1. Technical Field
The present invention generally relates to the field of electrowinning, also referred to as electrorefining or electroextraction, and more specifically to apparatus and processes for electrolytically removing metals from conductive liquids and facilitating recovery of such metals. Still more specifically, the invention relates to such apparatus and processes which provide for particulate cathode bed churning that is uncoupled to the flow of catholyte.
2. Description of Related Art
In an electrowinning process, a target metal ion is electrolytically removed from the solution phase of a conductive liquid and is deposited as a solid metal that can be recovered and potentially sold, either directly or after further purification. This approach to metal extraction is commonly used for the recovery and purification of several metals in the mining industry. Some of the metals that have been previously targeted for electrolytic recovery include copper, iron, lead and zinc. According to the U.S. Census Bureau's Economic Census, Mining Industry Series, in 2002 the electrowinning of copper from leaching operations accounted for more than 30% of copper ore mining production.
Some of the advantages of existing electrowinning methods include low operating costs, no chemical reagents required, solid product in low volume, high value form, reduced disposal issues and harmless byproducts (e.g., oxygen), potential for selectivity, and modular, scalable technology. These advantages are in many cases offset by common drawbacks such as medium to high capital cost, possible side reactions, and electrode fouling, corrosion, or other undesirable chemical reaction. Many existing systems suffer from awkward product removal, in some cases requiring equipment disassembly. Another common drawback of existing electrowinning technologies is low efficiency when processing dilute sources of metals. Most existing methods cannot be satisfactorily scaled up for processing large volumes of liquid and increased space velocities.
Conventional electrolytic technologies include standard plate and frame (2-dimensional) electro-deposition cells. These electrolytic systems are typically either planar or annular in design and operate at only moderate current densities, thus requiring large electrodes with correspondingly larger capital investments. Additionally, such cells generally suffer from large Ohmic losses due to significant interelectrode distances. Conventional plate and frame electrolytic technologies are not suitable for recovery of metals from dilute streams containing less than 1000 ppm metal.
Another type of technology in use today for electrowinning employs porous packed beds which provide large electrode areas (3-dimensional) and operate at higher current densities than typical 2-dimensional systems. Packed beds tend to become occluded by metal deposition, however, and are subject to shorting by interelectrode dendritic growth. Fluidized bed electrodes have also been extensively investigated. These electrolytic systems typically operate at much higher current densities than simple plate cells, and allow, to some extent, for electrode particle growth. Fluidized bed electrolytic systems still suffer from energy-intensive fluidization and dendrite growth leading to bed coagulation and process instability.
Spouted electrode technology (SET) comprises a modification of the fluidized bed technology, in which a jet or stream of electrolyte is introduced into the draft tube of an electrode forming a spout. The stream of electrolyte entrains and transports electrode particles in the particle bed up the draft tube, after which the particles are released onto the top of the particle bed and ultimately move to the bottom of the bed to be picked up again by the electrolyte stream. The electrode particles are in constant motion moving through the draft tube and into and through the particle bed during electrolysis. SET provides a number of improvements that overcome many of the limitations of previous systems, by providing a churning packed bed that resists dendrite growth, accommodates cathode particle growth, allows cathode bed removal/replacement without cell disassembly, and require less fluid transport energy than a fully fluidized bed. Although SET has been studied for many years, it has until recently been hampered by scale-up issues due to the typically annular design and unsuitability at metal concentrations below a few thousand parts per million.
U.S. Pat. No. 5,635,051 discloses a zinc electrowinning process using a mobile bed of particles. Motion of the bed is achieved by imposing a flow on the electrolyte solution in such a manner as to create a levitation region (a spout) in the cell distinct from, and preferably adjacent to, the moving packed bed. Favorable results in terms of production rate, current efficiency and energy consumption are said to be achieved by using a unique combination of design parameters and operating conditions achieved by selected ranges for particle size, current density, particle bed thickness and acid content of the electrolyte.
U.S. Pat. No. 6,298,996 discloses a spouted electrode and its use in electrowinning, in which cathode and anode chambers are separated by an inexpensive microporous membrane that prevents the cell from short-circuiting. The feed solution is jetted upwards into the cathode particle bed and fluidizes the central particles up through a spout tube. At the top of the cell the fluid velocity drops allowing the particles to fall back onto the bed. As with other conventional SET designs, the feed fluid and cathode bed particles are coupled in motion and flow path resulting in moderate removal rates and moderate control of selectivity. A prior art SET configuration, in which fluid and cathode bed flows are coupled, is shown in FIG. 1.
Most existing spouted electrode designs (both annular and planar) rely on the use of an electrolyte jet to churn the cathode bed. The linking or coupling of cathode spouting to electrolyte motion imposes certain inherent limitations on the technology. For instance, since a high flow rate is typically required to achieve spouting, operation parameter flexibility is limited. For heavy materials, such as plated metals, this can waste large amounts of energy to jet the electrolytic fluid fast enough to cause spouting. Also, most electrolyte passes through the spout and bypasses the bed so that excessive amounts of fluid transport occur and most fluid is simply recirculated without being treated. In many cases the result is low per pass removal rates, necessitating batch mode operation and increasing energy demands for pumping. Since most industrial metal recovery applications are more conducive to flow-through treatment, batch mode operation can be especially disadvantageous. The problem usually grows worse at lower metal concentrations where less bed motion is needed and higher electrolyte flow rates in the bed would improve efficiencies by reducing mass transport limitations. At metal concentrations of less than about 2000 ppm, the inherent problems of jetted SET are often glaringly apparent. Therefore, jetted SET is traditionally applied to only relatively concentrated solutions (e.g., 5000+ ppm), in which the above-mentioned limitations are less noticeable and higher operation current densities can be employed to offset the increased pumping demands.
Although there has been considerable advancement in the art of electrowinning, there is continuing interest in reducing process complexity, increasing separation efficiency and providing greater energy efficiency, in order to lower costs and boost productivity in such industries as metal mining, refining and recycling and for conserving natural resources and preventing pollution.